In electronic devices, such as personal digital assistants (PDAs), laptop computers, office automation equipment, medical equipment, and car navigation systems, touchscreens are widely used as their display screens that also serve as input means.
There are a variety of touchscreens that utilize different position detection technologies, such as optical, ultrasonic, surface capacitive, projected capacitive, and resistive technologies. A resistive touchscreen has a configuration in which an optically transparent conductive material and a glass plate with an optically transparent conductive layer are separated by spacers and face each other so as to function as a touchsensor formed of an optically transparent electrode. A current is applied to the optically transparent conductive material and the voltage of the glass plate with an optically transparent conductive layer is measured. In contrast, a capacitive touchscreen has a basic configuration in which a touchsensor formed of an optically transparent electrode is an optically transparent conductive material having an optically transparent conductive layer provided on a support and there are no movable parts. Capacitive touchscreens are used in various applications due to their high durability and high light transmittance. Further, touchscreens utilizing projected capacitive technology allow simultaneous multipoint detection, and therefore are widely used for smartphones, tablet PCs, etc.
Conventionally, as an optically transparent conductive material used for optically transparent electrodes of touchscreens, those having an optically transparent conductive layer made of an ITO (indium tin oxide) film formed on a support have been used. However, there has been a problem of low optical transparency due to high refractive index and high surface light reflectivity of ITO conductive films. Another problem is that ITO conductive films have low flexibility and thus are prone to crack when bent, resulting in increased electric resistance of the optically transparent conductive material.
A known optically transparent conductive material as an alternative to the one having an optically transparent conductive layer formed of an ITO conductive film is an optically transparent conductive material having, as an optically transparent conductive layer, a mesh pattern of a metal thin line on an optically transparent support, in which pattern, for example, the line width, pitch, pattern shape, etc. are appropriately adjusted. This technology provides an optically transparent conductive material which maintains a high light transmittance and which has a high conductivity. Regarding the shape of the mesh pattern formed of metal thin lines (hereinafter also written as metal pattern), it is known that a repetition unit of any shape can be used. For example, in Patent Literature 1, a triangle, such as an equilateral triangle, an isosceles triangle, and a right triangle; a quadrangle, such as a square, a rectangle, a rhombus, a parallelogram, and a trapezoid; an (equilateral) n-sided polygon, such as an (equilateral) hexagon, an (equilateral) octagon, an (equilateral) dodecagon, and an (equilateral) icosagon; a circle; an ellipse; and a star, and a combinational pattern of two or more thereof are disclosed.
As a method for producing the above-mentioned optically transparent conductive material having a metal pattern, a semi-additive method for forming a metal mesh pattern, the method comprising making a thin catalyst layer and a base metal layer on a support, making a resist pattern on the catalyst layer, making a laminated metal layer in an opening of the resist by plating, and finally removing the resist layer and the base metal layer protected by the resist layer, is suggested.
Also, in recent years, a method in which a silver halide diffusion transfer process is employed using a silver halide photosensitive material as a precursor to a conductive material has been proposed. In this method, a silver halide photosensitive material (a conductive material precursor) having, on a support, a physical development nuclei layer and a silver halide emulsion layer in this order is subjected to exposure with use of a pattern and then to a reaction with a soluble silver halide forming agent and a reducing agent in an alkaline fluid to form a metal (silver) pattern. The patterning by the method can reproduce uniform line width. In addition, due to the highest conductivity of silver among all metals, a thinner line with a higher conductivity can be achieved as compared with other methods. An additional advantage is that a layer having a metal pattern obtained by this method has a higher flexibility, i.e. a longer flexing life as compared with an ITO conductive film.
When an optically transparent conductive material having, on an optically transparent support, such a metal pattern as described above is placed over a liquid crystal display, the cycle of the metal pattern and the cycle of the liquid crystal display element interfere with each other, causing a problem of moire. In recent years, liquid crystal displays of different resolutions have been used, which further complicates the problem of moire.
As a solution to this problem, suggested in Patent Literature 2, Patent Literature 3, Patent Literature 4, and Patent Literature 5 is a method for suppressing moire by the use of a traditional random pattern described in, for example, Non Patent Literature 1. Also, disclosed in Patent Literature is a method for producing a metal pattern, the method comprising a step of calculating an evaluation value based on the quantified noise characteristic of the metal pattern to reduce granular noise which appears when the metal pattern is placed over a liquid crystal display.